If you place a via that has a span that goes to your middle plane, Xpedition will automatically connect the power or ground plane directly to the via padstack, if the net name is the same. You can define how the plane ties to your via padstack using the Plane Classes and Parameters dialog (e.g. buried under copper, tied at 45 degrees, etc..). You can see this connection in real time if you set your plane data state to be dynamic. This can be set in the Plan Assignments dialog.